1. Field of the Invention
The invention relates to a so-called tandem image forming apparatus including a plurality of image forming units provided in accordance with a plurality of toners having different colors and arranged in a predetermined direction to superimposedly transfer toner images of respective colors to a recording sheet being conveyed in the predetermined direction.
2. Description of the Related Art
Conventionally, a so-called tandem image forming apparatus including a plurality of image forming units arranged in one direction for forming toner images of respective colors of cyan, magenta, yellow and black to superimposedly transfer the toner images of the respective colors onto a recording sheet being conveyed in the arrangement direction is known (refer to Japanese Unexamined Patent Publication No. 2003-345101, for example).
This kind of image forming apparatus includes image forming units each including a photoconductive drum for bearing an electrostatic latent image and a developing device which is provided near the photoconductive drum. The image forming units are provided in accordance with respective colors of cyan, magenta, yellow and black. Each of the developing devices having toners of the respective colors develops an electrostatic latent image individually. After toner images of respective colors are transferred successively to a transferring belt, the toner images are transferred to a recording sheet and fixed by a fixing device.
Further, in the case of performing monochromatic printing is performed by such color image forming apparatus as described above, it is necessary to construct the image forming apparatus so that a developing operation is not executed with regard to the colors other than black e.g., cyan, magenta and yellow. Accordingly, each image forming unit is so constructed that the photoconductive drum and the developing device are driven by a common motor. Furthermore, in each of the image forming units corresponding to toners of cyan, magenta and yellow, is provided with a separating mechanism which makes and breaks a power transmission path between the motor and the developing device. In general, the separating mechanism is so constructed that it constantly transmits a driving force generated by the motor to the photoconductive drum constantly, and, on the other hand, it transmits the driving force to the developing device when needed (in the case of performing a color printing).
Further, Japanese Unexamined Patent Publication No. HEI10-319673 discloses an image forming apparatus including a plurality developing devices storing toners of respective colors of cyan, magenta, yellow and black arranged around a predetermined rotational axis and are held by a holding member, and a step motor which drives the holding member so as to rotate each developing device about the rotational axis to switch the developing devices for feeding toner particles to a photoconductive drum. In the image forming apparatus, the step motor executes microstep driving at the start time to prevent step-out and noise of the step motor occurred at the time of switching the developing device.
In the former tandem image forming apparatus, at the time when a state of performing a printing is switched from a monochromatic printing to a color printing and the separating mechanism makes a power transmission path between the motor and the developing device, a load by the developing device is applied to the motor instantaneously. Since the developing device has a relatively large weight, a great load is applied on the motor. Consequently, step-out of the motor occurs in some cases.
Further, in the latter image forming apparatus, the step motor executes the microstep driving at the start time. Consequently, a construction of a driving circuit becomes complicated, and a use of an expensive step motor which is capable of executing the microstep driving is needed.